The present invention relates to an output circuit, and particularly to a control circuit therefore.
Recently, semiconductor circuit devices utilizing LSI techniques have been improved so that they can be fabrricated with high density and, simultaneously, can be capable of operating faster. For example, MOS dynamic random-access memories, which are typical of such devices, have progressed to where 265k-bit memories are now commercially available, and simultaneously, access time has improved from 150 ns to 120 ns to 100 ns. When a semiconductor device is utilized in the deisgn of an apparatus or system, the efficiency of the system is often determined by the device which has a rapid access time and cycle time. For example, a memory can be made to execute a large number of processes, such as reading, writing, and so on, within a certain time by accelerating its cycle time. The cost per bit of a memory has been reduced, so that recently there has been a rapid increase in the use of memories in image processing. In this case, the memory must have a cycle time corresponding to the dot rate of a CRT in a display apparatus. In a memory used for this purpose, the important points are the data rate for reading and writing, and the effective width of the read-out data, rather than the access time.
In determining the data rate and the effective width of read-out data, an output circuit and a timing control circuit for controlling the output circuit have important roles. Namely, the timing control circuit controls the output operation and reset operation of the output circuit in response to a basic control signal. More particularly, the timing control circuit generates an activation signal a first predetermined delay time after the basic control signal becomes active. The active level of the basic control signal starts the output operation. The output circuit starts its output operation in response to the activation signal. Then, a second predetermined delay time after the basic control signal changes to an inactive level, the timing control circuit generates a reset signal to reset the output circuit.
Accordingly, the effective width of the output data corresponds to the period between the start of the activation signal and the reset signal. These signals are in turn dependent upon the activation period of the control signal, and thus upon the frequency of the control signal. The detailed feature of the above control technique is disclosed in U.S. Pats. Nos. 4,390,797 and 4,322,825.
However, if the repetition rate of the basic control signal becomes too fast, it becomes difficult to keep the effectiv width of the output data at a desired value. Namely, the width of the output data becomes remarkably short when the repetition rate is fast. Also, certain fluctuations must be considered in the characteristics of the control circuit.
Accordingly, it has been difficult to keep the effective width of the output data at the desired value for a high speed operation.
Thus, since the output data width is directly dependent upon the frequency of the basic control signal (strobe) in the memory circuit, an increased strobe frequency could reduce the output data width to a duration which is too short for subsequent application circuitry.